1. Field of the Invention
The present invention generally relates to punch and die sets used to join sheet metal as an alternative to welding. More specifically, this invention relates to a die having a specially formed cavity with peripheral interruptions, wherein the peripheral interruptions provide a limited amount of lateral extrusion of the sheet metal so as to improve the clinching strength of the resulting joined sheets.
2. Description of the Prior Art
Welding has long been a widely accepted method of joining sheet metal, particularly in the automotive and appliance industries. However, welding is recognized as having significant disadvantages, including the destruction of coating materials and its detrimental effect on corrosion-resistant alloys and metals which have undergone surface treatments. Welding has also become less desirable with the intensified concern for the environmental effects of its gases and flux residue.
In eliminating welding as a method for joining sheet metal of thicknesses between about 0.02 and 0.05 inches, various approaches have been suggested for using punch and die sets which are adapted to permanently join, or clinch, two or more sheets of metal. Generally, punch and die sets have some advantages over welding, including lower maintenance costs and lower power requirements to operate the device. However, the primary disadvantage with joints formed using punch and die sets is that they exhibit lower tensile and shear strength. It is inherent that the joint produced must be sufficiently strong for purposes of the particular application. As examples, where automotive hood and door panels or washing machine panels are to be joined, joints can be subjected to significant fatigue, shear and tensile loading. Accordingly, strength of the joint is a critical criterion for evaluating its suitability for a given application. In addition, the emphasis on providing leakproof joints precludes the use of conventional punch and die sets that pierce the metal sheets.
U.S. Pat. No. 3,579,809 to Wolf et al. is an early example of a die which was specifically adapted to provide a method of clinching metal sheets capable of producing a relatively high strength, leakproof joint. The method entailed positioning an anvil between a pair of die blocks resiliently biased toward the anvil. The anvil was recessed below the upper surfaces of the die blocks to provide a forming space between the die blocks. Two sheets of metal could then be placed upon the die blocks over the anvil, and a punch impacted against the sheets in axial alignment with the anvil such that the anvil served as a mechanical stop for the punch. The punch would form concentric bosses in the sheets without breaking through the joined metal. During the operation of the punch, the die blocks would slide apart against the opposing biasing force, allowing the sheets to plastically extrude laterally in all directions from the concentric bosses formed in the sheets. The lateral extrusions would serve to interlock the bosses, thereby forming a permanent joint able to withstand both shear and tensile loads in the plane of the sheets and in the axial direction of the bosses, respectively.
U.S. Pat. No. 4,459,735 to Sawdon taught a similar approach in which the sliding die blocks were replaced by pivoting die blocks. The die blocks were biased toward the anvil in a scissor-like fashion by a compression spring. As with the teachings of Wolf et al., the effect of allowing the die blocks to pivot was to allow lateral expansion of the bosses in the region abutting the anvil to interlock the sheets being joined. Sawdon placed particular emphasis on the importance of maintaining the thickness of the sheets at the center of the joint according to the formula: EQU T=0.2(1.2(M.sub.1 +M.sub.2));
where T is the total metal thickness at the center of the joint, and M.sub.1 and M.sub.2 are the top and bottom panel thicknesses, respectively, prior to the joining operation. Joints formed in adherence to this approach exhibited higher joint strength, but the approach necessitated that the press or punch apparatus be calibrated to take into account the original thicknesses of the panels. U.S. Pat. Nos. 4,757,609, 4,910,853 and 5,027,503 to Sawdon and U.S. Pat. No. 5,031,442 to Kynl introduced the use of an elastic band in place of the spring to resiliently bias the die blocks toward the anvil.
Though the Sawdon approach has been widely followed in the industry, the adherence to the above formula has posed a significant disadvantage in that panel thicknesses may vary sufficiently to require modification of the punch and die apparatus to achieve the preferred joint thickness. In addition, a primary disadvantage of Wolf et al. remained with the Sawdon approach in that mechanical movement of the die blocks was necessary to produce the lateral extrusion needed to lock the sheets together. The resulting die construction was rather large and cumbersome as well as expensive compared to using a one-piece anvil alone without movable die blocks.
In contrast, the approach taught by U.S. Pat. No. 4,584,753 to Eckold et al. avoided the use of mechanically movable die blocks by forming the die blocks as cantilevered members extending from a base to circumscribe the anvil. Accordingly, the resilient nature of the material from which the cantilevered members were formed allowed the cantilevered members to resiliently deflect outward during the operation of the punch to allow the lateral extrusion of the sheets to occur. Further adaptations of this principle are taught in U.S. Pat. Nos. 4,614,017, 4,658,502, 4,928,370, 4,972,565 and 5,046,228 to Eckold et al. However, the disadvantage of relying upon movable die blocks remained an important feature in the operation of the Eckold et al. punch and die apparatus.
An example of a punching process for clinching metal sheets which does not rely upon movable die blocks is illustrated in U.S. Pat. No. 4,911,591 to Oaks. Similar to the teachings of Sawdon and Eckold et al., Oaks also relies upon laterally extruding some of the sheet material to interlock the sheets. However, Oaks differs in that the interlocking occurs between a portion of one sheet which is extruded by a punch into a recess previously formed in the second sheet. This approach eliminates the need for die blocks which either slide, pivot or deflect relative to the anvil. However, a disadvantage to the teachings of Oaks is that the second sheet must first undergo an initial operation to displace a portion thereof to form the recess.
From the above discussion, it can be readily appreciated that the prior art does not disclose a device for a one-step joining operation for sheet metal in which the die is a unitary piece having no movable parts. Nor does the prior art teach or suggest a method which does not entail either preforming one of the sheets to form a recess therein, or resiliently expanding a portion of the die to accommodate the extrusion of one or more of the metal sheets to effectively and permanently clinch the sheets in a manner that provides a leakproof joint.
Accordingly, what is needed is a cost-efficient, one-piece die for use with conventional presses which, in cooperation with a conventional punch, is capable of joining together one or more metal sheets of various thicknesses without the need for the die to include movable parts and without the need to provide extensive preparation of the metal sheets prior to joining.